Academic journal article Genetics

Endogenous RNAi Pathways Are Required in Neurons for Dauer Formation in Caenorhabditis Elegans

Academic journal article Genetics

Endogenous RNAi Pathways Are Required in Neurons for Dauer Formation in Caenorhabditis Elegans

Article excerpt

ANIMALS are capable of undergoing physiological and behavioral changes to adapt to adverse environments, a process known as allostasis (Sterling and Eyer 1988). One such adaptation is the expression of polyphenism, or alternative developmental morphs, in a population of genetically identical organisms (Michener 1961; Mayr 1963). Sex determination in alligators (Woodward and Murray 1993), caste determination in insects (Nijhout 1998, 1999), seasonal differences in phenotypes of adult butterflies Precis Almana (Nijhout 1999), and growth of a "helmet-like" structure in Daphnia pulex (water flea) in the presence of predators (Brewer et al. 1999) are examples of polyphenisms in response to unfavorable environmental conditions.

Given that polyphenisms can occur in isogenic populations of animals, epigenetic mechanisms, such as RNA interference (RNAi) and DNA methylation, are hypothesized to regulate the expression of alternative phenotypic morphs in response to environmental conditions (West-Eberhard 2003; Wang et al. 2006; Kronforst et al. 2008; Kucharski et al. 2008; Moczek and Snell-Rood 2008; Hunt et al. 2010; Bonasio 2012; Humann et al. 2013). For example, in pea aphids, unwinged and winged morphs develop in response to favorable or unfavorable environments, respectively (Müller et al. 2001; Brisson 2010). Although genetically identical, winged and unwinged female populations exhibit differential DNA methylation patterns and transcriptional profiles of genes implicated in wing polyphenism (Brisson et al. 2007, 2010; Walsh et al. 2010). In addition, polyphenic transitions of locusts from solitary phase to gregarious (swarm formation) depends upon the differential accumulation of small RNAs in the two phases (Wei et al. 2009). Despite these examples, the molecular mechanisms that govern gene targeting and regulation by epigenetic pathways in response to environmental conditions are not well understood.

C. elegans is an excellent model system in which to study the molecular mechanisms regulating polyphenism as their developmental trajectory is determined by the environmental conditions experienced after hatching. Under favorable growth conditions, worms proceed through four larval stages (L1, L2, L3, and L4) to become reproductive adults. Under unfavorable conditions, such as low food availability, high temperatures, or high pheromone concentrations, L1 larvae will enter an alternative developmental stage called dauer (Cassada and Russell 1975; Golden and Riddle 1982; 1984a,b,c). Dauer larvae are stress-resistant and nonaging, and thought to facilitate dispersal in environmental conditions unfavorable for reproduction (Klass and Hirsh 1976; Larsen 1993; Frézal and Félix 2015). Once conditions improve, animals will exit dauer and resume continuous development as L4 larvae. Although adults that passed through, or bypassed, the dauer stage appear morphologically similar, we previously showed that C. elegans retain a cellular memory of their developmental history that is reflected in changes in gene expression, genome-wide chromatin states, and life history traits (Hall et al. 2010). Furthermore, we have shown that RNAi pathways are a major contributor to developmental history-dependent phenotypes in adults (Hall et al. 2013). However, the role of RNAi pathways in regulating environmentally induced phenotypic plasticity during early larval stages is unknown.

Environmental cues sensed by G protein coupled receptors (GPCRs) residing in the ciliary endings of sensory neurons differentially regulate the TGF-ß and insulin signaling dauer formation pathways (Fielenbach and Antebi 2008). Mutations in genes operating in these signaling pathways can result in dauer constitutive (daf-c) or dauer deficient (daf-d) phenotypes (Riddle et al. 1981; Vowels and Thomas 1992). Animals that form significantly fewer dauer larvae than wild-type in response to environmental stress are considered daf-d, while daf-c animals can form dauers even in the absence of environmental stress. …

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